It is known that unsaturated resins capable of free-radical polymerisation can be cured by the addition of photoinitiators and exposure to ultra-violet (UV) or visible radiation. The use of UV radiation in particular is very well established in the curing of thin films of resin used as surface coatings and inks. However UV curing is not suitable for thick coatings, or for glass-reinforced resins due to the poor penetration of the UV radiation which is strongly absorbed by the resin and the glass reinforcement if used. This means that cure is incomplete or takes an unacceptably long time to complete. Furthermore UV radiation is very dangerous to the eyes and skin and it is therefore necessary to use elaborate shielding and safety devices in order to safeguard the health of operatives. Such devices add considerably to the cost and inconvenience of the use of UV radiation.
In contrast visible light has none of these disadvantages. Furthermore it is readily available as natural sunlight, from tungsten filament bulbs, fluorescent tubes and so on, and is not harmful to the eyes or skin under normal conditions.
Systems for curing of resins with visible light have been described but have a number of disadvantages. For example, the components of some systems are extremely toxic or expensive, or show insufficient sensitivity to normal visible light and therefore require special and expensive light sources to perform satisfactorily.
It is already known that eosin plus a suitable reducing agent, such as ascorbic acid, can act as photoinitiator for the polymerisation of monomers in aqueous solutions in which water is part of the reaction chain. For example this system can be used to polymerise acrylamide (see G. Delzenne, S. Toppet, G. Smets J.Polymer Sci., 68; 347 (1960) and Ibid. Part A Vol. 2 1539 (1964)), in aqueous solution.
It has been previously suggested that a ketonic photosensitizer, especially .alpha.-diketones, plus a reducing agent capable of acting on the photosensitizer when the latter is in an excited state could be used for the curing of unsaturated polyester resins, (see GB PS No. 1,408,265). The reducing agent could in general be a tertiary amine. In a comparative example this patent discloses the use of rose bengal (tetrachlorotetraiodofluorescein, a polyhalogenated fluorescein) as photosensitizer, with unfavourable results. It is stated that if a pigment is present it must be absorbent of light wavelenghts different from those which excite the photosensitizer.
Benzoyl substituted-oxime carbonate esters (BOCE) have been claimed as potent U.V. photo initiators requiring low irradiation doses for curing resins (B.P. No. 1,537,921). However they are poor initiators when incorporated into unsaturated polyester resins and irradiated with visible light.